Thermal imaging or thermography is a recording process wherein images are generated by the use of imagewise modulated thermal energy.
In thermography three approaches are known:
1. Imagewise transfer of an ingredient necessary for the chemical or physical process bringing about changes in colour or optical density to a receptor element containing other of the ingredients necessary for the chemical or physical process followed by uniform heating to bring about the changes in colour or optical density. PA1 2. Thermal dye transfer printing wherein a visible image pattern is formed by transfer of a coloured species from an imagewise heated donor element onto a receptor element. PA1 3. Direct thermal formation of a visible image pattern by imagewise heating of a recording material containing matter that by chemical or physical process changes colour or optical density. PA1 BLUE DYE 1=Ceres Blue from Bayer AG [N,N'-2,6-diethyl-4-methylphenyl) -1,4, -diamino-anthraquinone] PA1 BLUE DYE 2=Orasol Blue from Ciba Geigy AG PA1 BLUE DYE 3=Reflex Blue R54 from Hoechst AG PA1 i) silver behenate/silver halide emulsion layer PA1 ii) protective layer
Thermographic materials of type 3 become photothermographic if a photosensitive agent is present which after exposure to UV, visible or IR light is capable of catalyzing or participating in a thermographic process bringing about changes in colour or optical density. Examples of photothermographic materials are the so called "Dry Silver" photographic materials of the 3M Company, which are reviewed by D. A. Morgan in "Handbook of Imaging Science", edited by A. R. Diamond, page 43, published by Marcel Dekker in 1991.
Particular sorts of thermographic film have a blue background, which can vary in tone and optical density, for example, for applications such as the printing of medical images for viewing in transmission. This blue background is not aesthetic having a number of functional purposes, for example: rendering the brownish tone of developed silver images blue/black, preventing over-exposure of the eyes of the viewer upon viewing in transmission with a view-box and improving image sharpness by reducing light scattering. Current practice is to achieve this blue background by incorporating one or more blue pigments or dyes into the support, thereby avoiding possible interference between the pigment or dye necessary to obtain the blue background and the other functional ingredients in the layer structure which makes up a thermographic material. However, this practice requires pigments and dyes which can withstand the high temperatures involved in kneading these dyes and pigments into the polymer (conventionally polyethylene terephthalate), in extruding the polymer to produce the polymer sheet, in stretching the polymer sheet and in conditioning the resulting support to reduce crimp upon later exposure to high temperatures during coating, drying, conditioning and use. Furthermore, this practice also requires the holding of an inventory of different sorts of blue background support as well as substantially colourless supports for producing a complete range of thermographic materials e.g. from graphics applications requiring a substantially colourless support to medical applications requiring a blue background support. The holding of such an inventory of different sorts of blue background support as well as substantially colourless supports incurs financial penalties due to additional storage and logistical requirements as well as increasing the possibility, easy in the subdued lighting required for the coating of photosensitive thermographic products, of using the "wrong" sort of support for the production of a particular thermographic material. This is in addition to possible financial penalties incurred by the possible necessary use of more expensive pigments and dyes in the colouring of the support due to the harsher conditions involved in the incorporation process. There is therefore a necessity for thermographic materials having different blue backgrounds, which can utilize a substantially colourless support.